1. Field of the Invention
The present invention relates to a copier, facsimile apparatus, printer or similar image processing apparatus and an ADF (Automatic Document Feeder) mounted thereon. More particularly, the present invention relates to a sheet-through type of ADF configured to convey one of a stack of documents to a reading position while separating it from the other documents, and an image processing apparatus loaded with the same.
2. Description of the Background Art
A sheet-through type of ADF customary with a copier includes a document table on which desired documents are stacked. The ADF conveys the top sheet from the stack to a glass platen included in the copier and then drives it out of the copier to a stack tray. One of conventional ADFs of the type described is constructed to enhance read productivity, i.e., the number of documents read for unit minutes by omitting a stop for registration, which is used to match a conveying position and a reading position at a position just preceding the reading position.
More specifically, the ADF not executing a stop for registration includes, e.g., separating and feeding means for feeding one of a stack of documents while separating it from the others and conveying means for conveying the document to the reading position. When the preceding document is being read, the following document is fed and caused to wait for a moment until the distance between the preceding document and the following document increases to a preselected value. When the distance reaches the preselected value, the following document is again conveyed to a preselected position upstream of the reading position at high speed, thereby reducing the distance between the consecutive documents.
Further, the above ADF includes a document sensor responsive to the trailing edge of the preceding document arrived at preselected position to thereby indicate that the distance between the preceding and following documents has reached the preselected value. The following document is again conveyed in response to the resulting output of the document sensor. With such a configuration, the ADF is capable of enhancing read productivity without executing the stop-for-registration operation, thereby implementing high-speed conveyance.
The conventional ADF, however, has the following problem left unsolved. Because the following document begins to be conveyed when the trailing edge of the preceding document reaches a preselected position, read productivity varies in accordance with, e.g., the slip ratio of conveyance. In fact, it is extremely difficult to maintain conveying speed constant because the coefficient of friction of conveying means decreases and because the coefficient of friction is dependent on the kind of a document. If read productivity is not constant, then document reading and image writing to be effected at the same time in, e.g., a copier are brought out of synchronism with each other.
More specifically, when the conveying means is again driven at the moment when the trailing edge of the preceding document is sensed, the distance between the two consecutive documents increases if slip is noticeable, although the distance may remain constant so long as substantially no slip occurs. An increase in the above distance results in the fall of productivity. Further, the slip degrades conveyance when the number of documents to be read increases.
Technologies relating to the present invention are disclosed in, e.g., Japanese Patent Laid-Open Publication No. 2001-160887.